How Lightning Damages Equipment Without Striking It
When lightning strikes near a building — not necessarily on it — the enormous current flow through the soil creates a rapidly changing magnetic field. This changing field induces voltages in any conductive loop within its range: power cables, data lines, earthing conductors, metallic pipes.
These induced transient overvoltages — surges — travel along service cables into the building and can reach amplitudes of several thousand volts. Modern electronic equipment, designed to operate at 5V, 12V or 230V, is destroyed instantly by these transients. The equipment does not need to be near the strike. It only needs to be connected to a conductor that runs near it.
According to insurance industry data, over 60% of lightning-related equipment damage is caused by induced surges rather than direct strikes. A building with a perfectly designed external LPS — ESE lightning rod, down conductors, earthing — but no surge protection on its service entries is still highly vulnerable to this damage mode.
What a Surge Protective Device (SPD) Does
A surge protective device (SPD) is connected in parallel with the service it protects. Under normal operating conditions, the SPD is essentially invisible — it passes the normal supply voltage without interference. When a transient overvoltage appears, the SPD clamps the voltage to a safe level (the voltage protection level, Up) and diverts the surge current to the earthing system.
The speed of this operation is critical. Modern SPDs respond in nanoseconds — fast enough to clamp the transient before it reaches connected equipment. The clamping voltage Up determines the residual stress on equipment: the lower the Up, the better protected the equipment. IEC 61643-11 defines Up categories; for sensitive electronics, Up should not exceed 1.5 kV.
The Three Types of SPD and Where to Install Them
Type 1 SPD is installed at the main distribution board — the point where the power service enters the building. It is designed to handle the direct lightning current component (tested to 100 kA with a 10/350µs waveform) and is mandatory when the building has an external LPS. Product selection follows network layout and coordination rules — refer to our SPD catalogue or request a project-specific specification.
Type 2 SPD is installed at sub-distribution boards and provides protection against switching surges and residual transients not fully absorbed by the Type 1. Tested to 40 kA with an 8/20µs waveform. See OSPD 101 for single-phase boards and OSPD 4011-3P for three-phase intelligent protection — plus the SPD overview.
Type 3 SPD is installed at the point of use — directly at the equipment being protected. It handles the residual transient not absorbed by Types 1 and 2 and provides the final layer of protection for the most sensitive devices.
Coordination is essential: The three SPD types must be selected so that each device's voltage protection level (Up) is lower than the previous stage. A Type 1 with Up = 4kV is useless if the Type 2 it feeds has Up = 6kV — the Type 2 would not activate until the voltage is already above the Type 1's clamping level. See our full system design guide.
Beyond Power — Data and Communication Lines
Power cables are the most common surge path, but data lines, ethernet cables, telephone lines and coaxial cables are equally vulnerable. A surge entering via a data cable can destroy network switches, servers, CCTV systems and building management controllers even when all power lines are protected.
Every conductive service entering the building — regardless of whether it carries power or signal — must be protected at the building entry point. This is the equipotential bonding principle: all conductive paths into the building must be bonded to the same earthing system at the point of entry, with appropriate SPDs on each active conductor.